Primary batteries are a cost effective disposable portable power source for a wide variety of consumer devices, although only a handful of standardized cell sizes (e.g., AA or AAA) and specific nominal voltages (typically 1.5 V) are typically utilized. Consumers often prefer and opt to use primary batteries for their convenience, reliability, sustained shelf life and more economical per unit price as compared to currently available rechargeable (i.e., secondary) batteries.
Many electronic devices, such as digital still cameras, are designed with relatively high power operating requirements. However, many of the historically-used primary battery chemistries (e.g., carbon zinc, alkaline, etc.) are not ideally suited for such high power applications. Furthermore, the need for specified nominal voltages significantly limits the selection of potential electrochemical materials, while the use of standardized cell sizes restricts the overall available internal volume available for active materials, safety devices and other similar elements and features typically required for consumer products. Smaller nominal voltage cells are preferred because they can be provided separately or in series, thereby giving device makers more design options and versatility. Designing a device to use primary batteries also presents significant cost advantages in comparison to secondary systems. Thus, 1.5 V primary systems, such as alkaline or lithium-iron disulfide systems, tend to be more prominent than higher voltage and/or rechargeable batteries.
Even among the 1.5V electrochemical systems (e.g., alkaline v. lithium-iron disulfide, etc.), the design considerations are significantly different. For example, alkaline and nickel oxy-hydroxide systems rely on an aqueous and highly caustic electrolyte that has a propensity for gas expansion and/or leakage. These issues lead to very different approaches in terms of selection of internal materials and/or compatibility with containers and closures as compared to non-aqueous systems. In rechargeable 1.5 V systems (note that lithium-iron disulfide systems are not currently considered suitable for consumer-based rechargeable systems), various highly specialized components and electrochemical and/or electrolyte compositions are used to best accommodate lithium ion charge/discharge cycling. Here, such high cost components are not a key design concern because secondary systems typically sell for a higher retail price than their primary battery equivalents. Ultimately, the discharge mechanisms, cell designs and safety considerations are, by and large, inconsequential and/or inapplicable between primary and secondary battery systems, and even among primary systems, it is difficult to adopt or interchange cell designs, materials' selections and the like.
For example, in 1.5 volt lithium-iron disulfide primary batteries, a discontinuity in the anode (which does not have an embedded current collector running along its entire length) can lead to a loss in the overall expected capacity of the battery. Thus, as described in United States Patent Publication Nos. 2003/0228518 and 2010/0310910, various mechanisms have been proposed to address and eliminate such “anode disconnects”. In contrast, in 3.0 volt lithium-manganese dioxide primary batteries, anode disconnects, such as those described in U.S. Pat. Nos. 5,965,290 and 6,391,488, can be deliberately engineered into the cell design to act as a safety feature (i.e., the portion of the anode which is connected to the battery terminal is disconnected from the rest of the anode to avoid short circuits if the battery is exposed to a forced discharge condition). Therefore, battery materials and designs should only be considered within the context of that particular battery system.
Although lithium-iron disulfide cells have distinct advantages for high power devices (as compared to primary alkaline cells), cell designs must also strike a balance between the cost of materials used, the incorporation of necessary safety devices and the overall reliability and capacity of the cell's design. Safety devices normally include venting mechanisms and thermally activated “shutdown” elements, such as positive thermal circuits (PTCs), while improvements to reliability primarily focus on preventing internal short circuits and maintaining the integrity of the electrodes, and especially the anode (as noted above). However, these safety and reliability elements occupy internal volume and/or require design principles that can be counterproductive to cell internal resistance, efficiency and overall discharge capacity.
Another challenge unique to lithium-iron disulfide systems relates to the fact that its reaction end products occupy substantially more volume than the original inputs, which leads to swelling of the electrode assembly as the battery discharges. In turn, this swelling can cause unwanted bulging (or, in extreme cases, splitting) of the cell container. The increased force within the electrode stack may also lead to short circuits if the separator is compromised and/or anode disconnects which reduce the amount of capacity actually delivered by the battery. In particular, the radial pressure caused by the expanding iron disulfide cathode forces cell designers to preferentially locate the anode lead on the outer- or inner-most wind of the jellyroll to minimize the effect that the non-active lead could have as the entire jellyroll is compressed.
Previous means of handling these problems include using stronger (often thicker) materials for the cell housing and inactive components within the cell. However, thicker inactive materials limit the internal volume available and thicker, more rugged electrodes accommodate fewer winds in the jellyroll, resulting in less surface area between the electrodes and comparatively lower battery performance at higher drain rates.
A number of approaches have been taken to strike an appropriate balance between optimal internal volume utilization and acceptable LiFeS2 cell capacity/performance. For example, a possible solution for problems created by swelling, disclosed in U.S. Pat. No. 4,379,815, is to balance cathode expansion and anode contraction by mixing one or more other active materials (such as CuO, Bi2O3, Pb2Bi2O5, P3O4, CoS2) with pyrite, although these additional materials can negatively affect the discharge characteristics of the cell, and the capacity and efficiency of the overall cell may also suffer. Other means of improving discharge capacity in LiFeS2 cell contemplate the use of thinner separators and/or specific cathode coating mixes and coating techniques, as disclosed in U.S. Patent Publication Nos. 20050112462 and 20050233214, or through the adjustment of interfacial input materials, as disclosed in U.S. Pat. Nos. 6,849,360 and 7,157,185. To the extent some of these solutions remove volume that could be occupied by active material in the battery, these solutions would not necessarily improve the battery's overall discharge capacity.
Ultimately, improvements to capacity represent a fundamentally sound battery design. That is, in order to deliver greater capacity, careful consideration must be given for the radial expansion forces and other dynamics at work in a discharging lithium-iron disulfide battery. For example, if the design provides inadequate thickness in the anode or the cathode current collector then the radial forces during discharge may compress the jellyroll to such a degree so as to cause a disconnect in one or both electrodes and, once this disconnect occurs, the battery may cease to deliver capacity regardless of whether the active materials have all been discharged. Similar situations arise with respect to the void volume (in the cathode coating and the interior of the cell as a whole), the electrical connections throughout the battery, the separator, the closure/venting mechanism for the battery and the like. Therefore, the capacity of a LiFeS2 cell is a significant metric for the overall viability and robustness of a cell design, particularly when the cell designer is limited to the use of a standard-sized (i.e., fixed volume) consumer battery (e.g., AA or FR6; AAA or FR03; etc.)
As a corollary to the capacity acting as a de facto metric for battery design, those skilled in the art will appreciate that design choices, and particularly the selection of specific components, must be made in consideration of the overall context of the lithium-iron disulfide system. A specific composition may have surprising, unexpected or unintended effects upon the other components and compositions within the cell. Similarly, in standard sized batteries, the selection of a particular element occupies volume within the container that might otherwise have been available for other elements. Thus, this interdependency of design choices necessarily means that any increase in capacity, and especially an increase that does not negatively impact the safety or performance of the battery in other regards, is much more than a simple act of adding more active materials. Similarly, the selection of inactive components, such as solvents, solutes, binders, conductors, polymers and the like, must necessarily be made in context of the exigencies of the situation, and isolating single items from an extensive list of possibilities is not by itself sufficient reason for an artisan to consider using in that particular combination.
Tests that simulate the actual discharge or use of the battery have particular relevance to evaluating cell designs. Typically, these simulated-use tests involve discharging the battery under specified discharge conditions (e.g., a constant load of 200 mA) continuously or in a predetermined cycle (e.g., discharge for a set number of minutes, followed by a rest interval of a set number of minutes) until the battery output voltage drops below a final “cut off voltage”. As used herein, tests that involve a cycle of discharge and rest intervals will be generically referred to as intermittent drain rate tests. Clearly, with any simulated use test, it is necessary to specify the discharge conditions, periods of time for discharge and the rest interval (if used) and the cut off voltage.